Method for formulating master alloy compositions for use in dispersion hardened compacts



United States Patent METHOD FOR FORMULATING MASTER ALLOY COMPOSITIONS FOR USE IN DISPERSION HARDENED COMPACTS Joseph P. Hammond, Knoxville, Tenn., and Krishna Kuman Sinha, Bombay, India, assignors to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Filed Feb. 25, 1965, Ser. No. 435,385

6 Claims. (Cl. 75206) The present invention relates to an improved method for formulating master alloy compositions suitable for use in making dispersion-hardened structures from a metal selected from tungsten, molybdenum and rhenium.

The discovery and development of the relatively new metallurgical technique of dispersion hardening has provided the art with a significant method for strengthening metals and alloys. In one sense, the phenomenon of improved strength in metals caused by dispersion of a second phase in a metal or alloy matrix is not new since heat treatment of steel and aluminum alloys to form strength-inducing dispersions has been known for some time. The significant new feature of dispersion hardening as it is applied in the present invention is the use of a new powder metallurgical technique for producing a dispersion, which is stable to high temperature. Efforts in this direction have demonstrated that many metallic systems are capable of being strengthened by an ultrafine dispersion of a hard insoluble phase. For example, because :of its very high melting point (2610 C.) high crystallization temperature (approximately 1100 C.) and high modulus of elasticity (47,000,000 pounds) and very low coefiicient of thermal expansion (5 X inch/ degree centigrade), molybdenum has attracted the attention of many researchers. Tungsten and rhenium are characterized by similar useful qualities. However, these metals are highly susceptible to oxidation, and have other shortcomings at elevated temperatures. The present invention is designed to improve the high temperature qualities of a metal selected from tungsten, molybdenum and rhenium and alloys thereof produced by powder metallurgical techniques by providing means for incorporating a stable strength-inducing dispersed phase within the structure of a matrix phase consisting of said selected metal.

It is generally agreed that in order to fabricate a potentially useful dispersion hardened structure, the dispersion must be chemically and physically stable with minimum solubility in the matrix phase. Moreover, the particle comprising the dispersed phase must be so fine and discrete as to produce a dispersion in a compact wherein the interparticle spacing of dispersed phase is of the order of 1 micron with an upper limit of 3 to 4 microns and preferably as small as 0.2 to 0.3 micron. The beneficial effect of a dispersion on the mechanical properties of a metal or alloy is primarily a function of the distance between the dispersed particles within a homogeneous matrix phase. That is to say, the closer the inter-particle spacing above a limit of about 0.10 micron, the greater will be the enhancement of mechanical properties of the alloy. It might be well to note here that desired inter-particle spacing is a function of the size and amount of the particles comprising the dispersed phase. It is, therefore, desirable, and an object of this invention to provide a method for the preparation of a master alloy containing a dispersed phase in which the inter-particle spacing of the master dispersion alloy is within the hereinbefore defined limits deemed desirable for enhancing the mechanical properties of the selected metal. It is also an object of this invention to control the preparation of a master dispersion alloy so that its interaparticle spacing can be 3,271,142 Patented Sept. 6, 1966 "ice reliably reproduced with a required degree of precision. Another object of this invention is to provide amethod for preparing a master alloy composition containing a dispersed phase with controlled inter-particle spacing. A still further object is to realize the aforementioned objects in connection with the preparation of a dispersion alloy in which the matrix phase consists of a metal selected from tungsten, molybdenum and rhenium and alloys thereof.

With these and other objects in mind, the present invention comprises forming an aqueous solution of a compound selected from molybdenum, tungsten and rhenium, mixing said solution with a liquid in which said compound is insoluble to effect formation of a finely divided colloidlike dispersion of said compound, mixing said dispersion with a liquid dispersion of a metal oxide having a negative free energy of format-ion at 1500" K. of greater than about 40 kilogram calories per gram-atom of oxygen, said oxide being further characterized in that it is mutually insoluble and non-reactive with the selected metal up to a temperature of about 2000 C., separating the liquid phase from the dispersed solid phases and thereafter drying the resulting solid. An additional aspect of this invention included with the broad statement of invention, as just recited, includes the steps of heating the resulting solid dispersion in a reducing atmosphere to convert the compound of the selected metal to its elemental form, and thereafter pressing the resultant metal-metal oxide dispersion to a compact having a desired density, said compact consisting of a continuous matrix phase of said metal element and a dispersed phase Within said matrix.

There are several well known methods which can be used in formulating master alloy composition suitable for use in fabricating dispersion-hardenedalloys. Among the more known methods of producing dispersion formulation are 1) oxidation of ultra-fine powders, (2) mechanical blending of fine powders of metal and oxide, (3) decomposition or chemical conversion of salts to derive refractory oxide coatings on fine metal powders, and (4) coating and co-deposition of powders, the object in point, in all cases, being to obtain an adequate degree of dispersion. Unless a homogeneous dispersion is achieved with ultra-fine particles to allow control of the inter-particle spacing in the finally fabricated structure within the previously defined narrow limits, the soughtfor improvements in mechanical properties will not be realized.

The first of these methods cannot be used to produce dispersion-hardened structures of said metals because of the low melting point of the oxides of molybdenum, tungsten and rhenium. Mechanical blending of metal powders is apparently a direct and obvious way to prepare the desired metal-metal oxide dispersion. However, control over the dispersion by this method is not good as a result of a tendency for the oxide dispersion to segregate under these conditions. However, metal powders in this required size range (an average particle size of the order of 1 micron) are not generally available on a commercial basis. It was thought that fine powders could be reduced to the required particle size by the ordinary ball milling operation. However, it has been found that the ultimate particle size achievable by dry grinding or ball milling is limited by agglomeration and flocculation tendencies of the powder during grinding. In addition, the purity of the end product was adversely affected. In the chemical conversion and coating schemes, the tendency of the particles in a dispersion to coalesce and/or agglomerate restricts realization of the fine dispersions needed in compacts.

In citing these methods of preparing master alloy compositions, it is not meant to imply that such methods are not entirely successful in producing dispersion-hardened materials. What is intended, and what our experience has shown, is that these methods are subject to several troublesome variables which make it difiicult to reproduce, in a reliable manner, a powder mixture of the dispersed phase and matrix phase which can be compacted to form a homogeneous matrix phase containing the required sp acing between the dispersed particles distributed Within the matrix. In marked contrast, the concept of our invention for forming a master alloy composition is simple and direct and reproducible. It involves the initial formulation of a stable colloidal-like dispersion of the dispersion phase and the precursor matrix phase. By colloidal-like, we mean the development of a finely divided dispersion or suspension within a liquid carrier dispersion medium, in which the particles do not display a tendency to coagulate or aggregate, but whose size may not fall within the colloidal range, or exceed somewhat the colloidal dimensions.

The ideal material for the dispersant is one that is completely inert with respect to the matrix phase of the finally fabricated alloy compact. The two phases of the compact should be mutually insoluble and exhibit little, if any, tendency to react with one another. Oxides, nitrides, carbides, silicides and inter-metallic compounds have been found useful as the dispersant phase. Of these, oxides are generally the most satisfactory, and the most desirable oxides are those having a high negative free energy of formation, of the order of 40 kcal. per gram-atom of oxygen at l500 K. Among some of the most useful oxides which may be used as the dispersant phase are the refractory metal oxides such as ziroonia, thoria, alumina, beryllia, titan-ia, magnesia, and rare earth type oxides such as yttria. The art is fully familiar with techniques for forming the aqueous dispersions of these oxides by standard colloid chemistry techniques, such as by peptization. Other techniques are described in the literature, for example, in Weiser, Inorganic Colloid Chemistry, vol. 2, Hydrous Oxides and Hydroxides.

The formation of a dispersion of a metal or salt of a metal having particles in the colloidal range selected from molybdenum, tungsten, and rhenium is a far more complex problem. Moreover, once such a dispersion is formed, it must be mixed, according to this invention, with the metal oxide sol, and the resultant metal oxidemetal compound dispersion must be stable for at least a short period of time with-in which the liquid dispersing medium may be effectively separated or removed. The gist of this invention then lies in the discovery and formulation of a stabilized liquid colloid-like dispersion of a compound selected from tungsten, molybdenum and rhenium combined with a stable dispersion of a metal oxide sol. The concept of this invention further includes the discovery that inter-particle relation of the liquid dispersion can be retained even after the liquid dispersion medium is removed and the compound of the selected metal is converted to metal. Thus, a second aspect of the invention includes the steps of removing the liquid carrier from said liquid dispersion, drying the resultant solid cake, heat-ing the resultant cake in a reducing atmosphere to convert the compound of the selected metal to the metal in elemental form, and then consolidating the resulant solid dispersion into a compact.

It should be emphasized here that the preparation of a colloidal mixture of the selected compound to serve as the matrix as the source of the metal matrix phase of the compact is not enough to serve the purposes of this invention, for the resultant colloidal solution must not only be stable per se, but should be stable when mixed with the metal oxide sol. Thus, for example, it is known that molybdic acid has a marked tendency to go over to the colloidal state from an aqueous solution thereof. However, when such a colloidal suspension is mixed with a thoria sol, for example, the combined suspension or dispersion is not stable for a sufiicient period of time to allow separation of the liquid phase, and hence form a uniformly mixed solid dispersion. The reason for this seems to be that in that particular case, the aqueous solution of molybdic acid is so strongly acid as to affect the stability of the oxide sol. Hence, the selection of the metal compound from which the matrix phase is to be derived must be governed by this criterionnarnely, that it should not affect the stability of the metal oxide sol with which it is mixed. Moreover, molybdic acid does not form colloids of suificiently high concentration to be useful in the present process.

We have found that of the several molybdenum, tungsten, and rhenium compounds, ammonium molybdate, ammonium tungstate, and ammonium rhenate meet all the requirements necessary to form a stable metal-metal oxide sol from which the master alloy formulation can be made. In each case, an aqueous solution of the selected metal compound is formed, and then mixed with a solvent to which the compound is relatively insoluble, i.e., relative to water. For the most part, the solvent which serves this purpose includes the lower alkyl alcohols, and the most suitable of these are methyl alcohol and ethyl alcohol. In practice, a quantity of alcohol is added to a solution of the selected metal compound in sufiiicent amounts to create a dispersion of the compound. A sol of a selected metal oxide is then mixed with the metal compound dispersion, and the resultant metal oxide-metal compound dispersion is thoroughly mixed, such as in a Waring blender. The liquid phase of the dispersion is then separated from the solid phases by filtration, evaporation, centrifugation, flocculation, vacuum evaporation, or other solid-liquid separatory techniques. The resultant metal oxide-metal compound composite is then reduced in a hydrogen atmosphere to effect reduction of the metal compound to metal. The final metal-metal oxide powder mixture is then ready for consolidation by standard powder metallurgical techniques to form a compact.

Having described the invention in general terms, together with outlining essential parameters thereof, the following example will elucidate the method in somewhat more concrete terms.

Example A dispersion of thoria (ThO was effected in a molybdenum matrix as follows: .To ml. of a 0.3 molar ammonium mo-lybdate (NH Mo O -4H O solution was added ml. of ethyl alcohol. The ammonium molybdate solution was rapidly stirred during addition to effect rapid mixing of the two liquids. A very fine, almost colloidal precipitate of ammonium molybdate formed when all the alcohol was added. A thoria sol was prepared by steam denitrating thorium nitrate crystals at about 400 C. The resulting fine oxide was readily peptized in distilled water in the presence of a trace amount of nitric acid. 20 ml. of the thoria sol was added to the ammonium molybdate sol. After a thorough mixing of the oxide sol and molybdate dispersion in a Waring blendor, the liquid phase was removed by evaporation while continuing the mixing up to the point of dryness. The metal oxide-molybdate cake thus obtained was converted to oxide and then metal plus oxide in a two-stage hydrogen reduction operation. In the first stage of reduction, the matrix material is reduced at 500 to 600 C. to M00 In a second stage of reduction at 1000 to 1l0O C., the molybdenum oxide was converted to molybdenum metal, the thoria remaining unaffected throughout. The resulting molybdenum-thoria metal powder was vacuum hot pressed at 1400 C. to form 1.25-inch diameter by 1 inch pellets. Photomicrographs of hot pressed specimens show a near optimum dispersion of the oxide particles. The dispersed particles of ThO were about 0.1 micron in size and spaced 0.1 to 0.3 micron apart. Hot hardness tests conducted up to 10 00" C. showed a marked superiority of these composites over a molybdenum-oxide compact from master alloy formulation produced by the coprecipitation technique.

For the sake of comparison, a ThO -Mo compact was pressed from a master alloy formulation prepared by a co-precipitation technique as follows: To an aqueous solution of oxalic acid and ammonium molybdate was added, while stirring, a solution of thorium nitrate in concentrated nitric acid. The resulting white precipitate of molybdic acid and thorium oxalate was stirred, fil tered, dried, and finally reduced in hydrogen in two stages-first at 600 C. and then at 1100 C. Pellets were pressed out from this metal-oxide powder as with other powder (removed from colloidal process) and sintered before examination of their microstructure. The dispersant particles tended to be agglomerated and were not uniformly spaced.

It will thus be seen that the colloidal process of formulating the metal-metal oxide master alloy composition is an extremely efiicacious way of achieving the fine particles required of the matrix phase, in order to insure the formation of a homogeneous matrix phase, and of the dispersed phase, in order to obtain the required homogeneous distribution defined interapartiole spacing.

In the example, we have shown the colloidal-like suspension is produced by adding a non-solvent to the ammonium molybdate solution. In some cases, where the selected metal compound contains waters of hydration, a stable colloid-like dispersion can be formed simply by mixing the hydrated compound with a non-solvent. Thus, a stable colloidal-like suspension of ammonium molybdate can be efi'ectuated simply by mixing with a volume of ethyl alcohol and ball milling. The resultant suspension can then be used in the same manner as described in the example.

Having thus described our invention, we claim:

1. A process for producing a compact containing a reiractory metal oxide uniformly dispersed within a matrix of a metal or alloy selected from tungsten, molybdenum, and rhenium which comprises mixing a colloidal-like dispersion of a water-soluble compound of said selected metal with a colloidal dispersion of a refractory metal oxide which is stable and non-reactive with said selected metal at any temperature up to about 2500 C., separating the liquid phase from the colloidal mixture to obtain a solid dispersion of metal oxide-metal compound, heating the resultant solid cake in a reducing atmosphere to selectively reduce the metal compound of the selected metal and then compacting the resultant powdered dispersion of metal oxide in the selected metal to a densified structure.

2. The method according to claim 1 wherein the densified structure is sintered or hot pressed to a desired density.

3. The method according to claim 1 wherein the selected metal is molybdenum and the metal compound from which it is derived is ammonium molybdate.

4. The method according to claim 1 wherein the selected metal is tungsten and the metal compound from which it is derived is ammonium tungstate.

5. The method according to claim 1 wherein the selected metal is rhenium and the metal compound from which it is derived is ammonium rhenate.

6. The method according to claim 1 wherein the selected metal is an alloy comprised from molybdenum, tungsten, and rhenium.

References Cited by the Examiner UNITED STATES PATENTS 3,019,103 1/1962 Alexander et al. 25230l.l

L. DEWAYNE RUTLEDGE, Primary Examiner.

R. L. GRUDZIECKI, Examiner. 

1. A PROCESS FOR PRODUCING A COMPACT CONTAINING A REFRACTORY METAL OXIDE UNIFORMLY DISPERSED WITHIN A MATRIX OF A METAL OR ALLOY SELECTED FROM TUNGSTEN, MOLYBDENUM, AND RHENIUM WHICH COMPRISES MIXING A COLLOIDAL-LIKE DISPERSION OF A WATER-SOLUBLE COMPOUND OF SAID SELECTED METAL WITH A COLLOIDAL DISPERSION OF A REFRACTORY METAL OXIDE WHICH IS STABLE AND NON-REACTIVE WITH SAID SELECTED METAL AT ANY TEMPERATURE UP TO ABOUT 2500*C., SEPARATING THE LIQUID PHASE FROM THE COLLOIDAL MIXTURE TO OBTAIN A SOLID DISPERSION OF METAL OXIDE-METAL COMPOUND HEATING THE RESULTANT SOLID CAKE IN A REDUCING ATMOSPHERE TO SELECTIVELY REDUCE THE METAL COMPOUND OF THE SELECTED METAL AND THEN COMPACTING THE RESULTANT POWDERED DISPERSION OF METAL OXIDE IN THE SELECTED METAL TO A DENSIFIELD. STRUCTURE. 